“After identifying some issues we instantly activated Monzo Stand-in – our fully independent backup bank,” they told the BBC.
“As a result customers can still make payments with their card, withdraw cash, freeze their card, send and receive bank transfers and more.”
“We are working on resolving this as soon as possible.”
Many attempting to open the app shortly after 15:00 GMT on Thursday were met with a notice telling them “we’re experiencing issues”.
This said the app “will not be fully functional” but other services, such as viewing account details and moving money between accounts, would be.
However, some users attempting to access the app took to social media to complain to Monzo that they could not view funds, recent payments or make bank transfers.
In posts seen by the BBC, some X users also told Monzo they had been unable to use their card or withdraw money.
The BBC has asked Monzo for comment about these complaints.
The company has more than 14 million personal and business customers across the UK.